89-91 Turbo II Boost creep vs mods

 

First lets understand how the turbo and wastegate controls boost. When exhaust gas flows out of the engine, it goes through the turbo blades which spins the turbo. As the turbo spins the boost goes up and forces more air into the engine. More air into the engine means more exhaust gases, which come out into the turbo to make it spin faster again. See a problem here? In a few secs you would have infinite boost. This is where the wastegate comes it. It sits in parallel to the turbo in the exhaust path. So when exhaust comes out of the engine, it can either go through the turbo, or be bypassed through the wastegate. During light throttle and low boost the wastegate stays closed. This causes all air through the turbo so you get the best spool up time and response. As soon as the wastegate sees full boost. It opens up to let some air bypass the turbo to maintain the set boost, and not going over.

The biggest problem with moding almost any turbo car is boost creep. (Different then a boost spike) Creeping is when the boost goes higher then what it is set at while on the throttle. the higher the gear, the more it will creep as there is more load on the engine, so 5th gear is worse, 4th is bad, 3rd mgiht creep alittle..Even though the boost is being raised which you might want to do, Creep is not a good thing. They correct way to raise boost with a boost controller, not creep. The problem here is the hole in the wastegate isn't that big. Adding an aftermarket exhaust and intake makes the engine flow so much more air. When all that added air tries to go through the wastegate, it can't all fit through, so more goes through the turbo instead. Causing the boost to raise above the set amount. The only ways to lower the boost creep are to remove the turbo and port the waste gate hole bigger (best way for most cars), restrict your airflow through and out of the engine (smaller exhaust or intake). But of course that will lose power across the whole rpm band... The last way is with an external wastegate, which is much bigger. But this requires a new exhaust manifold and a new turbo, as well as the wastegate. This is the best way to control. But very expensive!! and not needed for most cars.

 

This is my Experience with getting boost creep with certain mods. This was done on my car, stock engine and turbo with 100k miles. Different cars with different condition engines and turbos will differ, as will 87-88 turbos which have worse wastegates expect 3-5psi more on each section with a 87-88. So this is just a guild to use for you to use. Results might differ on yours.

The S5 (89-91) turbo 2s have a 5.5 (or so) psi spring in the wastegate, same as the 87-88. However the S5 also have an electronic boost controller which raises the boost to 7.5-8psi. The controller is mounted on the driver side under the Intercooler. It's a small box with 2 vac lines and a wire connecter.

In stock form they should be seeing 7.5-8psi. (although probably won't get it in first gear.) If you remove the Vac line from the turbo and wastegate and connect them together (bypassing the stock boost controller) you should get 5-5.5psi.

 

Ok so now the stock car is too slow, Time to start moding :) Here is the order of what I did, and my outcomes:

(before starting moding please make sure everything is tuned up and running strong!, plugs, wires, fuel filter, timming etc need to be good. I take no responsabilty if you damage or blow your engine while running these mods or that much boost)

-Removed Stock airbox and added a K&N cone filter onto the AFM and built a cold air box. Better response and spool up, still 8psi. Not enough difference to effect creep past 8psi.

-Installed FD fuel pump and rewired it to get full voltage. Bigger pump and rewire very recommanded for more then stock boost levels.

-bypassed stock controller by running a vac line right from turbo to wastegate. I would recommend this before decent mods, I really think it will help control creep much better bypassed. Add an aftermarket boost controller if you want. Ended up with 5psi in 2nd, But got around 6psi in 3rd-5th. So the K&N is enough to start creeping past 5psi.

-Added full 3" racing beat exhaust and FCD. Got out on the highway and slowly crept to 11psi in 4th gear before letting off. More then I want!!

-Removed K&N cone and reinstalled stock airbox. Test drive shows 9psi max. Perfect! and turbo spools pretty fast :) But boost starts dying around 6000rpm or so. Down to 5-6psi at redline. (will do it when stock too) managed to hit 10psi a few times up big hills. Still OK.

-Added manual boost controller. Now instead of the boost slowly raising from 5-9 as it creeps, it shoots straight to 8/9. much better mid range!

-removed FCD and installed G-force ECU chip. Exhuast temps droped from 800-810C max after long WOT uphill, to 760C. Probably to rich. But better to be safe!

-Replaced the RB presilencer with a custom 3" magnaflow high flow cat. Boost hitting 8psi max (would probably go lower if I turned down the boost controller). also quite a bit quieter.

-reinstalled K&N cone filter. Better response and midrange. Still creeping to 8 psi max. Need to make cold air box

-Added 3" TID mod. Best response and midrange so far!! and crazy intake noise. max 9psi and it holds it all the way to redline. Beat my top speed up a local huge hill :D Still need to build a cold air box for it.

 

That's were I sit now, can't wait to dyno it!